The present invention relates to the field of electrical circuitry, and more particularly relates to layered circuit structures such as multilayer circuit boards, to components and methods utilized in fabrication of such structures and to methods of making the same.
Electrical components are commonly mounted on circuit panel structures such as printed circuit boards. Circuit panels ordinarily include a generally flat sheet of dielectric material with electrical conductors disposed on a major, flat surface of the sheet or on both major surfaces. The conductors are commonly formed from metallic materials such as copper and serve to interconnect the electrical components mounted to the board. Where the conductors are disposed on both major surfaces of the panel, the panel may have via conductors extending through the dielectric layer so as to interconnect the conductors on opposite surfaces. Multilayer circuit panel assemblies have been made heretofore which incorporate plural, stacked circuit panels with additional layers of dielectric materials separating the conductors on mutually facing surfaces of adjacent panels in the stack. These multilayer assemblies ordinarily incorporate interconnections extending between the conductors on the various circuit panels in the stack as necessary to provide the required electrical interconnections.
Electrical components which can be mounted to circuit panel structures include so-called “discrete” components and integrated circuits which include numerous transistors on a single chip. Chips of this nature can be mounted to elements commonly referred to as “chip carriers” which are specialized circuit panel structures. A chip carrier may be incorporated in a package which is mounted to a larger circuit board and interconnected with the remaining elements of the circuit. Alternatively, the chip can be mounted directly to the same circuit panel which carries other components of the system. This arrangement is ordinarily referred to as a “hybrid circuit”. Relatively large circuit panels are commonly made of polymeric materials, typically with reinforcement such as glass, whereas very small circuit panels such as those used as semiconductor chip carriers may be formed from ceramics, silicon or the like.
There have been increasing needs for circuit panel structures which provide high density, complex interconnections. These needs are addressed by multilayer circuit panel structures. The methods generally used to fabricate multilayer panel structures have certain serious drawbacks. Multilayer panels are commonly made by providing individual, dual sided circuit panels with appropriate conductors thereon. The panels are then laminated one atop the other with one or more layers of uncured or partially cured dielectric material, commonly referred to as “prepregs” disposed between each pair of adjacent panels. Such a stack ordinarily is cured under heat and pressure to form a unitary mass. After curing, holes are drilled through the stack at locations where electrical connections between different boards are desired. The resulting holes are then coated or filled with electrically conductive materials, typically by plating the interiors of the holes to form what is called a plated through hole.
It is difficult to drill holes with a high ratio of depth to diameter. Thus, the holes used in assemblies fabricated according to these prior methods must be relatively large and hence consume substantial amounts of space in the assembly. These holes ordinarily extend from the top or bottom of the stack. Even where interconnections are not required in the top or bottom layers, space must be provided for holes to pass through these layers so as to provide needed interconnections in the middle layers. Accordingly, substantial amounts of the available surface area on the panels must be allocated to the holes and to accommodate tolerance zones around the holes. Moreover, the electrical interconnections formed by depositing conductive materials in such drilled holes tend to be weak. The drilling method and the general nature of the laminates used therein is described, for example in Doherty, Jr., U.S. Pat. No. 3,793,469; and Guarracini, U.S. Pat. No. 3,316,618.
Various alternative approaches have been proposed. Parks, et al., U.S. Pat. No. 3,541,222; Crepeau, U.S. Pat. No. 4,249,032; Luttmer, U.S. Pat. No. 3,795,037; Davies, et al., U.S. Pat. No. 3,862,790, Fox, U.S. Pat. No. 4,954,878, and Zifcak, U.S. Pat. No. 4,793,814 all relate generally to structures which have metallic or other conductive elements arranged at relatively closely spaced locations on a dielectric sheet with the conductive elements protruding through the dielectric sheet in both directions. Such a sheet may be sandwiched between a pair of circuit boards and the circuit boards may be clamped or otherwise held together so as to provide mechanical engagement between conductive elements on the adjacent faces of the circuit boards and the conductive elements of the composite sheet. In each of these arrangements, the conductive elements, the composite sheet or both is resilient or malleable so as to provide for close engagement between the conductive elements of the composite sheet and the conductors on the circuit boards.
Beck, U.S. Pat. No. 3,616,532 and Dube, et al., U.S. Pat. No. 3,509,270 describe variants of this approach in which resilient elements are used with a fusible solder. These elements are mounted on insulating boards which are then stacked between printed circuit layers. The assembly is heated so as to melt the solder, thereby freeing the resilient elements so that the resilient elements and solder cooperatively form an interconnection between the adjacent circuit boards.
Evans, et al, U.S. Pat. No. 4,655,519 describes a connector with numerous strip-like contact springs disposed in holes in a flat dielectric body, together with other spring elements. The ends of the strips protrude from opposite surfaces of the body. These are adapted to compress when electronic elements are engaged with the body surfaces, so that the ends of the strips engage pads on the electronic elements. Walkup, U.S. Pat. No. 5,167,512 discloses a further system using springs disposed in holes in a dielectric body.
Grabbe, U.S. Pat. No. 5,228,861 describes a connector having a sheet-like dielectric body with numerous generally X-shaped spring contact elements, each having four arms, lying on a first side of the sheet. Two arms of each X-shaped element are bent inwardly toward the sheet, and extend through holes in the sheet so that the tips of these arms protrude above the second, opposite face of the sheet. The other two arms are bent away from the sheet, and hence protrude from the first surface. When the connector is placed between circuit panels, each X-shaped element is compressed between mating pads of the circuit panels, causing the bent arms to flatten and causing the tips of the arms to wipe the surfaces of the pads. After engagement, the contact is maintained by the resilience of the arms.
Bernarr, et al., U.S. Pat. No. 4,548,451 describes a connector or interposer having a sheet-like elastomeric body with crushable protrusions extending outwardly from oppositely-directed surfaces. Tabs formed from a metal-coated flexible film extend on both surfaces of the body, and overlie the protrusions. The tabs on opposite sides are connected to one another by vias. When the interposer is engaged between circuit panels, the tabs and posts are crushed between contact pads on opposing panels, and the tabs assertedly wipe the pads for more effective contact. The tabs are maintained in engagement with the pads by the resilience of the elastomeric sheet and the posts; there is no permanent bond formed.
Patraw, U.S. Pat. Nos. 4,716,049; 4,902,606; and 4,924,353 describe microelectronic connection schemes using deformable contacts protruding upwardly from a substrate. Each contact has a dome-like tip and a plurality of legs extending downwardly from the tip to the substrate. These contacts are formed by selective deposition of aluminum on pedestals of a fugitive material such as potassium chloride or a photoresist using a shaped mask. The pedestals are removed after deposition.
Dery, et al., U.S. Pat. No. 4,729,809 discloses the use of an anisotropically conductive adhesive material disposed between opposing sublaminates, the adhesive composition having sufficient conductivity across the relatively small spaces between conductors on adjacent layers to form an electrical interconnection therebetween, but having low conductivity across the relatively large spaces between adjacent conductors on the same surface so that it does not produce an unwanted lateral interconnection along one surface.
Berger, et al., U.S. Pat. No. 4,788,766 uses conductor bearing circuit lamina having hollow, eyelet-like via structures, each such via structure having a rim protruding vertically from the surrounding structure. Each such via structure is provided with a thin layer of a conductive bonding material. In making the multilayer structure, dielectric bonding films are interposed between the circuit bearing lamina. The dielectric films have apertures in locations corresponding to the locations of the eyelet structures, in the adjacent circuit bearing lamina. Thus, the upstanding rims of the eyelet structures can bear upon one another when the assembly is forced together under heat and pressure. The layers of conductive bonding material on the rims of the abutting eyelets are said to form bonds between the abutting eyelet structures.
Co-pending U.S. patent application Ser. No. 08/277,366 of Thomas H. DiStefano, et al., which is assigned to the same assignee as the present application, discloses an interposer having deformable contacts protruding upwardly from its surfaces. Each contact has a central axis normal to the surface and a peripheral portion adapted to expand radially outwardly from the central axis in response to a force applied by a pad on an engaged circuit panel. Thus, when the circuit panels are compressed with the interposers, the contacts expand radially and wipe across the pads. The wiping action facilitates bonding of the contacts to the pads, as by conductive bonding material carried on the contacts themselves.
Other structures for forming multilayer electronic assemblies are taught in Dux, et al., U.S. Pat. No. 5,224,265 and Ehrenberg, et al., U.S. Pat. No. 5,232,548, which use sublaminates made by depositing a dielectric material onto a perforated metal sheet, as by vapor-phase polymerization or electrophoretic bonding, to form a dielectric sheet with vias. The vias are filled with a flowable joining material such as a metal-loaded polymer. These structures are stacked and heated to join the vias into unitary vertical conductors.
Other multilayer assembly systems using flowable conductive materials to join structures in stacked elements are disclosed in Bindra, et al., U.S. Pat. No. 5,129,142. Still further improvements are disclosed in U.S. Pat. No. 5,282,312 of Thomas H. DiStefano, et al. The '312 Patent discloses as background certain lamination techniques or methods of making multilayer circuit assemblies using flowable conductive materials.
Despite these and other efforts in the art, there are needs for still further improvement.